Egyptian police arrested a leading secular activist on wanted for trial during a raid on the office of a rights group in Cairo, the NGO said Thursday.
Police detained six people at the office during the raid Wednesday night, but released all except activist Mohamed Adel hours later, the Egyptian Centre for Economic and Social Rights said.
Amnesty International called on Egyptian authorities to investigate allegations that the six staffers were "tortured or otherwise ill-treated in custody this morning".
A statement by the watchdog also called on the authorities to reveal the whereabouts of Adel and to ensure that non-governmental organisations can work unhindered in Egypt.
Adel, a prominent secular activist, is on trial with two other activists for violating a new protest law that restricts demonstrations.
He had been on the run and was being tried in absentia.
The Egyptian Centre for Economic and Social Rights, founded by former presidential candidate Khaled Ali, has supported striking factory workers and defended activists at trials.
Adel is being tried along with Ahmed Maher and Ahmed Douma, leaders of the April 6 movement that helped to topple strongman Hosni Mubarak in 2011.
The verdict in their trial, on charges of holding an unauthorised and violent protest in Cairo late last month, is expected next week.
Their arrest and trial are seen by secular activists and rights groups as widening a government crackdown which had previously targeted only the Islamist supporters of ousted president Mohamed Morsi.
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"Officers stormed the office around 11:30 (Wednesday night, 2130 GMT) and arrested everyone who was there," said the group's executive director Nadeem Mansour, adding that Adel was a former employee of the NGO.
Police ransacked the office and took away three computer hard drives, according to an AFP correspondent who saw the office on Thursday.
The room targeted in the raid was used as a media and documentation centre, and the group said one of those arrested, Mostafa Eissa, headed its documentation unit.
Two computer screens were knocked over, chairs had been toppled, and cables were strewn across the wooden floor, the AFP correspondent reported.
Interior ministry officials were not immediately available for comment.
Mahmoud Bilal, a lawyer working with the NGO and one of the six arrested, said that around 50 plainclothes policemen carried out the raid.
They "blindfolded us, handcuffed us and took us to an unknown location. The entire night we were made to stand with our backs to a wall, all the time blindfolded and handcuffed," he told AFP after being released along with four others.
Police raided several foreign NGOs in 2011 after prosecutors accused them of receiving illicit funding from abroad.
A court later sentenced 43 NGO employees, including US citizens who had left the country, to terms ranging from one to five years in prison.
Police have launched a blistering crackdown on Morsi's Islamist supporters since his ouster on July 3, with more than 1,000 people killed in street clashes and thousands arrested.
They have also begun to target secular activists who opposed Morsi and now criticise the military-installed government.